Rh blood group phenotyping is required both before blood transfusion and to determine if potential Rh incompatibility exists in pregnant women. Normally the Rh phenotype is established by a hemagglutination test. However, a limiting factor with available reagents is that human IgG anti-Rh is unable to induce direct red cell agglutination. Although IgM monoclonal antibodies can directly agglutinate Rh-positive red blood cells, anti-Rh IgMs are frequently polyreactive and give false positives. In the Phase I SBIR application the investigators developed a novel human blood typing reagent consisting of a human IgM-like (polymeric) recombinant antibody containing the constant regions of human IgG and the variable regions of human anti-Rh monoclonal antibodies. The proposed polymeric anti-Rh IgG has been constructed and expressed in myeloma cells. This novel antibody is correctly assembled and secreted and most important: it directly agglutinates Rh-positive red blood cells. For the Phase II SBIR the following specific aims are proposed. First, to produce two formulations of polymeric anti-Rh IgG. Second, to determine the fine specificity and affinity of polymeric anti-Rh IgG. Finally, to compare the polymeric anti-Rh IgG with commercially available anti-Rh reagents. It is expected that the anti-Rh polymeric IgG will make Rh phenotyping faster, more accurate and less expensive.